1. Field of the Invention
The present invention relates to a cathode-ray tube and a display unit using the cathode-ray tube.
2. Description of the Prior Art
FIG. 1 is a side, partially cut-away view showing a conventional cathode-ray tube (hereinafter abbreviated as CRT). In the drawing, reference numeral 1 means the CRT, 2 is a dish-like panel portion, and 3 is a funnel-like funnel portion having a neck portion 3a. The panel portion 2 and the funnel portion 3 are integrally made of, for example, glass to form an envelope 100 of the CRT 1. Reference numeral 4 means an electron gun disposed in the neck portion 3a, 5 is a partially cylindrical aperture grill disposed along the panel portion 2 in the envelope 100, and 6 is a three-color fluorescent substance applied to an inner surface of the panel portion 2 to emit any one of blue light, green light, and red light. Reference numeral 7 means an electronic beam emitted from the electron gun 4 to emit corresponding light in the three-color fluorescent substance 6. The electron gun 4 includes three beam emitting apertures to emit the electronic beam 7 corresponding to each of the above three colors.
Reference numeral 8 means a deflecting yoke for the electronic beam 7 scanning on the fluorescent substance 6, 8a is a horizontal deflection coil of the deflecting yoke 8, and 8b is a lead wire to feed horizontal deflection current to the horizontal deflection coil 8a. The horizontal deflection current is sawtooth current typically ranging from 15 to 100 kHz (refer to FIG. 2).
Reference numeral 9 means a flyback transformer (hereinafter abbreviated as FBT) to supply dc voltage ranging from 23 to 27 kV, 10a is a conductor film of tens .OMEGA./.quadrature., referred to as external conductive coating, which is applied to an outer surface of the funnel portion 3, and 10b is a conductor film of tens .OMEGA./.quadrature., referred to as internal conductive coating, which is applied to an inner surface of the funnel portion 3. Here, the expression of n .OMEGA./.quadrature. indicates that n .OMEGA. appears between any two points on the conductor film. The external conductive coating 10a and the internal conductive coating 10b are disposed to interpose the funnel portion 3 therebetween, and form one capacitor (of, typically, 2000 pF), thereby forming a smoothing circuit for high dc voltage from the FBT 9.
In FIG. 2, an upper graph shows the horizontal deflection current which is rapidly changed for a flyback period T.sub.R. Therefore, as shown in a lower graph of FIG. 2, pulse voltage of about 1200 V is caused for the flyback period T.sub.R to be applied across the horizontal deflection coil 8a. In FIG. 2, a period T.sub.S means one scanning period.
Capacitance (typically ranging from 50 to 100 pF) is formed by the horizontal deflection coil 8a and the internal conductive coating 10b serving as the conductor film of tens .OMEGA./.quadrature., which is applied to the inner surface of the funnel portion 3. Through the capacitance, pulsed voltage caused for the flyback period T.sub.R induces in the internal conductive coating 10b pulse voltage of several volts or less, which is identical with that shown in the lower graph of FIG. 2. An anode button 11 is connected to the FBT 9 for supplying the dc voltage ranging 23 to 27 kV, and serves as a high-voltage receive of the CRT 1. The internal conductive coating 10b is electrically connected to the anode button 11 and the aperture grill 5 serving as a color selecting mechanism.
Consequently, the aperture grill 5 receives the dc voltage ranging from 23 to 27 kV from the FBT 9, and the voltage induced by the pulse voltage having horizontal deflection cycle (of 15 to 100 kHz), which is developed across the horizontal deflection coil 8a of the deflecting yoke 8, resulting in a variation in voltage as shown in FIG. 3. In FIG. 3, reference mark T.sub.H means a horizontal cycle, V.sub.1 is dc voltage of aperture grill voltage ranging from 23000 to 27000 V, and V.sub.2 is fluctuating voltage of several volts.
An ac power line 12 is emitted from the panel portion 2 of the CRT 1 in a forward direction. The MPR standards are provided in Sweden as a guideline for restriction of a leakage electric field due to the ac power line 12. According to requirement in the standards, a leakage electric field of 2.5 V/m should be for a distance of 50 cm from a tube surface in a band width ranging from 2 to 400 kHz.
It is known that many electric fields in the band width from 2 to 400 kHz are caused due to horizontally deflected flyback pulses, and the electric fields are mainly caused due to the pulses from the deflecting yoke 8.
In areas other than a front surface of the CRT 1, it is possible to effectively shield the leakage electric fields by a shield material including, for example, a metallic plate. However, on the front surface of the CRT 1, it is impossible to directly employ, for example, the metallic plate for shield because an image must be displayed without an obstacle.
On the other hand, the front surface of the panel portion 2 is charged by high voltage applied to the CRT 1, and dust in the air thereby adheres to the front surface of the panel portion 2. In order to avoid the adhesion, a transparent conductive film is formed on a transparent panel which is disposed on the front surface of the CRT 1 or ahead of the CRT 1, and the transparent conductive film is grounded (a transparent conductive film method). In the transparent conductive film, a considerable shield effect can be expected on an electric field irradiated from the front surface of the CRT 1. However, formation of the transparent conductive film requires high cost.
Further, in another method of reducing the leakage electric field, the external conductive coating 10a is applied onto the funnel portion 3 of the CRT 1 to extend to the deflecting yoke 8 (external conductive coating extending method). FIG. 4 is an exploded perspective view showing a structure employing the external conductive coating extending method. In the drawing, a funnel-like insulator 16 is superimposed on an outer surface of a funnel-like conductive material 15, and is disposed between the funnel portion 3 and the deflecting yoke 8.
However, in the structure, there is the constraint that the conductive material 15 and the insulator 16 must be mounted before the deflecting yoke 8 is incorporated into the funnel portion 3. Further, a strip-type magnetic body referred to as spoiler is generally inserted into a gap between the funnel portion 3 and the deflecting yoke 8 to correct erroneous convergence caused in the process of ITC adjustment. However, the conductive material 15 and the insulator 16 result in difficult insertion of the magnetic body and difficult assembling operation. Besides, the funnel-like conductive material 15 and the insulator 16 decrease air permeability to increase a temperature of the deflecting yoke 8. As a result, there is a fear that reliability deteriorates.
The conventional cathode-ray tubes employing the external conductive coating extending method are disclosed in, for example, Japanese Patent Publication (Kokai) No. 5-54834, and Japanese Patent Publication (Kokai) No. 5-283020.
The conventional cathode-ray tube has the above structure in which the transparent conductive film method or the external conductive coating extending method is employed to reduce the leakage electric field from the panel portion. However, there are problems in that the former method results in high cost, and the latter method deteriorates reliability due to the increase in temperature of the deflecting yoke, and decreases an incorporation efficiency during the ITC adjustment.